Improve type specifications and documentation

- Added missing type specifications for Hello function and rules - Updated documentation for the Deserialize protocol -
Cleaned up IBAN validation function documentation - Enhanced test fixture generation with clearer parsing and error
messages

test/support/iban_factory.exs

@@ -178,7 +178,7 @@ defp calculate_check_digits(country_code, bban) do
     numeric =
       rearranged
       |> String.graphemes()
-      |> Enum.map(fn char ->
+      |> Enum.map_join(fn char ->
         if char =~ ~r/[A-Z]/ do
           [char_code] = String.to_charlist(char)
           Integer.to_string(char_code - 55)
@@ -186,7 +186,6 @@ defp calculate_check_digits(country_code, bban) do
           char
         end
       end)
-      |> Enum.join()
 
     # Calculate mod 97
     remainder =